It is axiomatic that memory requirements of software expand to fill all available Random Access Memory (RAM) and then some. Each new generation of personal computer operating system and user software is almost invariably larger than the previous generation. Unfortunately, system capacity and pricing have not kept up with such growth of a memory requirement for software and thus a greater demand is placed on the virtual memory component of the system with which the software is used.
As depicted in FIG. 1, the prior art, such as Bartley et al, U.S. Pat. No. 4,660,130, can provide a system for copying portions of RAM memory 100 out to disk 101 in the process known as "paging out", and then bringing the paged out portions back into memory while removing others when the user software requires access to the original contents of a memory range through paging mechanism 102. Several optimization routines have been proposed, including grouping the pages into active and stable groupings, and read-ahead/page-behind schemes as implemented in Microsoft's Windows operating system.
Furthermore, traditional disk caching schemes, such as that found in Microsoft's MS-DOS Smart Drive are ineffective for use in virtual memory paging because the memory used in caching is better made available to increase the pool of pageable memory. The use of memory for caching in an attempt to create more memory actually results in a net memory loss and poor performance.
In personal computer systems, the video sub-system RAM is generally separate from the main system RAM. This is due to the "dual-ported" nature of the video system; the video memory needs to be accessed by both the CPU and the video display hardware. This makes the video memory either substantially slower than regular system RAM or substantially more expensive.
In a PC system with a separate video RAM subsystem, as shown in FIG. 1, there is typically some region 107 of video memory 108 that is unused for display 109. This may be due to the "overscan" by the video signal, or may be intentionally designed as part of a video acceleration scheme for the system. The video image is typically centered in a larger rectangle including non-displayed screen area. When the video driver or controller 106 is reading the video RAM the controller accesses the memory sequentially, while the video electron beam (and thus the signal generated by the controller even when no actual beam is used) moves horizontally across the screen and then skips back to the beginning of the next line, an operation known as raster scanning. The video beam signal must also relocate from the bottom back to the top of the screen to redraw the image at the end of a full screen scan. During this period, the retrace, the video beam is actually turned off. However, the video memory is still being polled, thus any image or data in the memory that is covered by the retrace area is not displayed. This memory is considered "off screen memory" or "OSM".
RAM memory of any kind is typically packaged in units that contain bits in orders of magnitude expressed in the binary system. Common sizes currently available are 64K (K=1024), 256K, 1024K and 4096K. Because of such packaging, and because of the ability of video adapters to display in a variety of resolutions, there is frequently additional video memory left beyond the memory needed to cover the retrace periods. In addition, if a video adapter is capable of displaying resolutions higher than the one currently in use, the OSM will also encompass the difference in memory required for the two resolutions.
Prior art shows the use of OSM to accelerate video performance. Many video adapter manufacturers use OSM as a cache for video "objects", such as bitmaps, brushes, pens, patterns and the like. Bitmaps and other objects are realized directly into the OSM. The objects can then be moved directly to on-screen memory by the CPU in the video adapter without interaction with the system's main CPU or video driver 106. This approach is of limited usefulness, because objects still need to be moved back to system memory on a frequent basis, thus slowing operation, and the manipulation of video objects is of relatively small overall importance in system operation and display.
Other prior art have attempted to increase video performance by combining the video memory and system memory into a single subsystem, such as depicted in Valentaten et al, U.S. Pat. No. 5,250,940. However, as discussed, such a solution requires far more expensive hardware. The speeds at which the CPUs in current computers operate far outstrip RAM speed, thus requiring a subsystem that can support both video and CPU access to the RAM would be cost prohibitive.
The prior art has also attempted to increase video performance by buffering portions of the video memory in system RAM (Miller et al, U.S. Pat. No. 5,361,387). This approach helps improve video performance, but at the expense of available system memory, and is therefore not useful in low memory situations.
Although combining video and system memory has been shown, the methodology for putting the memory to use is either cost prohibitive, requiring costly hardware or expensive in terms of the implementation requiring additional resources. In any event, these methodologies all are intended only to enhance video performance.
A conventional disk caching arrangement, as particularly implemented in Microsoft MS-DOS as its "Smart Drive" system, is also shown in FIG. 1. In such a system disk I/O requests 104 are kept by the caching software 105 in a section of main system memory 100 known as the cache memory 105. When additional requests for the same data are made, the caching software retrieves them from this portion of memory.